Many satellite and terrestrial optical communication systems require transmission of analog optical signals. A straightforward way to address this need is to modulate the amplitude (AM) of an optical carrier. This approach, however, suffers from a poor Signal to Noise Ratio (SNR). It is well known that broadband modulation schemes, which utilize higher bandwidth than that of the transmitted waveform, may improve the SNR over that achieved with AM. Pulse position modulation (PPM) is one of such techniques. In PPM, a shift in the pulse position represents a sample of the transmitted waveform, as shown in FIG. 1. It can be shown that for a given power, SNRPPM∝SNRAM(tp/τ)2, where tp is the spacing between un-modulated pulses and τ—the pulse duration, respectively. See H. S. Black, Modulation Theory, D. Van Nostrand (1953).
The implementations of PPM for optical communications require new techniques for generating trains of optical pulses whose positions are shifted in proportion to the amplitude of a transmitted waveform. Typically a bandwidth of Δf=1–10 GHz and higher is of interest for inter-satellite communications. Since pulse repetition frequencies (PRF) of 1/tp>2 Δf are required for sampling a signal of bandwidth Δf, GHz trains of picosecond (ps) pulses are required for realizing the advantages of PPM. For example, an optical inter-satellite link designed to transmit waveforms with Δf=10 GHz bandwidth requires sampling rates of PRF=1/tp≧2Δf=20 GHz. By employing 1–2 ps-long optical pulses, a 30 dB gain is realized over an AM system with equal optical power.
Optical PPM offers large SNR improvements in power-starved optical links. This technology, however, requires development of new types of optical PPM receivers. One optical PPM receiver based on top hat pulse generation (THPG) has been proposed. See S. I. Ionov, “Detection of optical analog PPM streams based on coherent optical correlation”, U.S. Pat. No. 6,462,860. See also S. I. Ionov, “Optical top hat pulse generator”, US Published Patent Application No. 2003/0219195, and “PPM demodulator based on PM NOLM with improved conversion efficiency”, U.S. patent application Ser. No. 10/735,071 filed Dec. 12, 2003 which is based upon 60/488,540 filed Jul. 18, 2003.
The present disclosure describes a significantly simpler approach to PPM decoding. Because of its simplicity, the proposed device is expected to be more robust. The technology alluded to above utilizes fiber-based designs. The major drawback of the fiber-based design is in its complexity. The previous receivers were based on non-linear optical loop mirrors (NOLM) that require careful balancing and adjustments. They also need a number of EDFAs and optical filters with flat-dispersion.
The reader is also directed to U.S. patent application Ser. No. 10/701,378 filed Nov. 3, 2003 which relates to a PPM demodulator based on the gain dynamics of a semiconductor optical amplifier (SOA).
This disclosure relates to a different implementation of a PPM demodulator based on interferometric schemes involving SOAs.